On NMT Search Errors and Model Errors: Cat Got Your Tongue?

Felix Stahlberg, Bill Byrne

Abstract
We report on search errors and model errors in neural machine translation (NMT). We present an exact inference procedure for neural sequence models based on a combination of beam search and depth-first search. We use our exact search to find the global best model scores under a Transformer base model for the entire WMT15 English-German test set. Surprisingly, beam search fails to find these global best model scores in most cases, even with a very large beam size of 100. For more than 50% of the sentences, the model in fact assigns its global best score to the empty translation, revealing a massive failure of neural models in properly accounting for adequacy. We show by constraining search with a minimum translation length that at the root of the problem of empty translations lies an inherent bias towards shorter translations. We conclude that vanilla NMT in its current form requires just the right amount of beam search errors, which, from a modelling perspective, is a highly unsatisfactory conclusion indeed, as the model often prefers an empty translation.
Anthology ID:
D19-1331
Volume:
Proceedings of the 2019 Conference on Empirical Methods in Natural Language Processing and the 9th International Joint Conference on Natural Language Processing (EMNLP-IJCNLP)
Month:
November
Year:
2019
Address:
Hong Kong, China
Editors:
Kentaro Inui, Jing Jiang, Vincent Ng, Xiaojun Wan
Venues:
EMNLP | IJCNLP
SIG:
SIGDAT
Publisher:
Association for Computational Linguistics
Note:
Pages:
3356–3362
Language:
URL:
https://aclanthology.org/D19-1331
DOI:
10.18653/v1/D19-1331
Bibkey:
Cite (ACL):
Felix Stahlberg and Bill Byrne. 2019. On NMT Search Errors and Model Errors: Cat Got Your Tongue?. In Proceedings of the 2019 Conference on Empirical Methods in Natural Language Processing and the 9th International Joint Conference on Natural Language Processing (EMNLP-IJCNLP), pages 3356–3362, Hong Kong, China. Association for Computational Linguistics.
Cite (Informal):
On NMT Search Errors and Model Errors: Cat Got Your Tongue? (Stahlberg & Byrne, EMNLP-IJCNLP 2019)
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PDF:
https://preview.aclanthology.org/naacl24-info/D19-1331.pdf
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